This invention relates in general to electrophotography, and in particular, to an electrophotographic imaging member having an overcoating layer.
In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles (toner) on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
In the imaging process, it is necessary to clean residual toner from the surface of the photoconductive insulating layer prior to repeating another imaging cycle. One common method of cleaning is blade cleaning.
Elastomer blade cleaning of photoreceptors is conceptually simple and economical, but raises reliability concerns in mid- and high-volume applications due to apparent random failures. Such random failures justify the reluctance to include blade cleaners in higher volume machines with or without some back-up element.
Alternative cleaning techniques used in higher volume applications include the use of magnetic, insulative and electrostatic brushes. However, such cleaning techniques are also subject to specific or timed failures. These failures include, but are not limited to, photoreceptor filming and cometing. Specific failures may, in part, be related to the materials package, e.g., the toner and any additives contained with the toner. These types of blade and cleaning failures can be quite predictable
One random failure mode of a cleaning blade may be due to inherent variations or flaws in the material of the blade, which allow stresses and strains with extended copying to locally fatigue the edge of the blade. An additional random failure mode can be local or image related enhancements or reductions in blade/photoreceptor friction which cause unacceptably large tuck-under of a doctor blade edge. A large enough tuck or break in the blade/photoreceptor seal can permit residual toner and other debris to pass under the blade. This not only decreases cleaning efficiency, for example by increasing background, but in severe cases can result in catastrophic system failure.
A number of methods have been implemented or proposed to enhance blade/photoreceptor contact properties. One method includes agitation of the blade against the photoreceptor to prevent build-up of material along the contact seal. Another method includes addition of redundant members, such as disturber brushes to loosen or collect debris which might otherwise stress the blade element. These methods increase the mechanical complexity and the cost of the cleaning assembly, and are thus undesirable.
Another method for enhancing blade/photoreceptor contact properties includes the addition of lubricants to the toner, photoreceptor and/or blade. However, this method increases the materials complexity and introduces compatibility problems.
A further proposal for enhancing blade/photoreceptor contact properties is by roughening of the photoreceptor surface to reduce the blade/photoreceptor contact area, and thus the blade friction. This method may also introduce compatibility problems depending on how the roughened surface is introduced. For example, particulate additives to the bulk of the transport layer can degrade electrical and/or mechanical properties. Surface asperities can be worn away in normal machine copying, limiting any cleaning benefit. Surface roughening can also have direct adverse effects such as the introduction of sites against which toner may become lodged. Photoreceptor surface roughening can also inhibit cleaning by allowing the blade to pass over toner and other surface debris.
U.S. Pat. No. 4,647,521 to Oguchi et al discloses the addition of amorphous hydrophobic silica powder to the top layer of a photosensitive member. The silica is of spherical shape and has a size distribution between 10 and 1000 Angstroms.
U.S. Pat. No. 4,784,928 to Kan et al discloses an electrophotographic element having two charge transport layers. An outermost charge transport layer or overcoating may comprise a waxy spreadable solid, stearates, polyolefin waxes, and fluorocarbon polymers.
One of the most common "predictable" or non-random blade cleaning failures is photoreceptor cometing. This type of failure is generally encountered and resolved during program development. Photoreceptor cometing involves material, including toner particles, which becomes impacted onto the photoreceptor and adheres with such force that the material cannot be removed by the cleaning elements. Additional debris, including untransferred toner residue and developer and/or toner additives, may become jammed against the asperity. Repeated passes and extended copy can lead to the build-up of elongated crusty deposits in front of the asperity which eventually print out as spots on the copy, i.e., the comets.
Various strategies have also been implemented or proposed to deal with this type of blade cleaning problem, including those enumerated above. Additional approaches to the resolution of cometing problems include the elimination of the material which impacts or builds up in the tail, the inclusion of additives which lubricate and/or scavenge the offending material, and the development of a photoreceptor surface which resists toner impaction and/or cometing.
The prevailing opinion as to the origin of comets in blade systems is that localized tucks in the cleaning edge allow the toner particles or comet heads to be compressed into the photoreceptor. Thus, cometing and the more random type of blade cleaning failures may be related.
In some electrophotographic imaging systems, a raster output scanner has been employed to create images on the photoreceptor. Raster output scanners create or write images in accordance with the image content of an image signal. Typical raster output scanner systems include xerographic based systems where the images are written on a photoreceptor. In such devices, the moving photoreceptor, which has been previously charged, is exposed line to line by a high intensity beam of electromagnetic radiation, such as a laser, that has been modulated in accordance with an input signal. The modulated beam is focused by suitable optical elements to a point on the photoreceptor by a scanning element such as a rotating multi-faceted polygon. As a result, latent electrostatic images representative of the input image signal are created on the photoreceptor and are thereafter developed by the application of a suitable toner thereto. The developed images are then transferred to copy sheets and fixed to provide permanent copies.
Some problems, however, are associated with the use of a raster output scanner. Coherent light of a wavelength of about 6500 .ANG.ngstroms to about 8500 .ANG.ngstroms, when internally reflected between the top and some bottom surface of the imaging member, produces an interference pattern in light absorbed by the photoreceptor. The bottom internal surface usually is a conducting metal ground plane, but may also be an interface at the charge generation layer, the charge transport layer, or other layers in the photoreceptor. The variation in absorption results in a variation in photodischarge, which may print out as an objectionable pattern in a xerographic printer, particularly under conditions of partial discharge. The resulting pattern resembles grain in wood laminates, and is accordingly referred to as plywood.
It is known that the interference pattern may be eliminated by diffuse reflection, for example by roughening the surface of the internal reflective layer. For example, roughened metal substrates may be used. However, roughening of an evaporated metal layer is difficult.
U.S. Pat. No. 4,904,557 to Kubo discloses that dispersions of finely divided inert particles within the photoreceptor can cause sufficient light scattering to eliminate plywood. As indicated above, and also in U.S. Pat. No. 4,904,557, such additives can degrade electrical and/or mechanical properties of the imaging member. This patent also suggests that interference fringe patterns can be avoided by surface roughening achieved by controlling spraying conditions or by grinding. This surface roughness is represented by an average roughness RZ of ten points over a reference length of 2.5 mm, and is equal to or larger than 1/2 of the wavelength of the light source employed for image formation.
Overcoating layers for electrophotographic imaging members have been proposed for a number of differing reasons. U.S. Pat. No. 4,912,000 to Kumakura et al discloses a protective layer for an electrophotographic photoreceptor. The protective layer comprises a product of uncatalyzed hydrolysis of a composition essentially consisting of at least one specific epoxy silane compound, at least one specific alkyl alkoxy silane compound, and at least one specific amino silane compound. The protective layer protects the photoreceptor from wear due to friction with paper and cleaning members.
U.S. Pat. No. 4,469,771 to Hasegawa et al discloses an electrophotographic light-sensitive member having a protective coating. The protective coating consists of an organic high polymer-containing Lewis acid.
U.S. Pat. No. 4,587,189 to Hor et al and U.S. Pat. No. 4,588,666 to Stolka et al disclose multilayer photoconductive imaging members. The imaging members are provided with an exposed hole transport layer comprised of aryl amine compounds. The '666 patent to Stolka discloses a hole transporting molecule comprised of alkoxy derivatives of tetra phenyl biphenyl diamine.
U.S. Pat. No. 4,615,963 to Matsumoto et al discloses an electrophotographic imaging member having a photosensitive layer which is applied as a paste-like mixture, dispersion, or solution, and quenched to a frozen state under high vacuum under which drying is performed. The drying method is provided so as to avoid formation of coarse particles in the photoconductive composition.
U.S. Pat. No. 4,537,849 to Arai discloses a photosensitive element having a roughened selenium-arsenic alloy surface. The outer photoconductive surface is roughened by direct mechanical grinding (polishing). A roughness of less than or equal to 3.0 micrometers laterally and from 0.1 to 2.0 micrometers in height is disclosed for reducing adhesion of transfer paper or toner.
U.S. Pat. Nos. 3,992,001 and 4,076,564 to Fisher disclose roughened imaging surfaces of a xerographic imaging member. Roughening of the photoreceptor surface is accomplished indirectly by first chemically etching a substrate. The substrate is then uniformly coated with photoconductive material which conforms to the surface in such a way that the substrate roughness is reproduced on the photoconductive surface. The level of roughness may be from 3 to 5 micrometers laterally with a 1 to 2 micrometers height or from 10 to 20 micrometers laterally with a 1 to 2 micrometers height.
The Kodak ColorEdge (TM) photoreceptor (introduced in 1988) is provided with a highly and specifically textured surface. The texturing is obtained by placing a "dot screen" on internal layers of the photoreceptor followed by overcoating with a charge transport layer. The final surface conforms closely to the dimensions of the "internal" asperities. The photoreceptor is cleaned by a fur brush cleaner and thus the roughened surface is not believed to be engineered to assist cleaning.
U.S. Pat. No. 4,904,557 to Kubo discloses an electrophotographic photosensitive member comprising a photosensitive layer having a surface roughness of ten points over a reference length of 2.5 millimeters. The particular surface roughness is provided to prevent an interference pattern appearing at image formation, and for preventing black dots appearing at reversal development.
U.S. Pat. No. 4,693,591 to Takasu et al discloses an image bearing member having a maximum surface roughness of 20 micrometers or less, and an average surface roughness which is less than or equal to two times a toner particle size. Takasu et al have a limitation on the amplitude of the roughness, but do not disclose the particular wavelength between peaks.
U.S. Pat. No. 4,804,607 to Atsumi discloses an overcoat layer which is a film-shaped inorganic material coating the surface of a photosensitive layer. The overcoat layer is formed such that the rough surface is provided having convexities and concavities with a maximum depth difference of 0.5 to 1.5 micrometers. The convexities and concavities are formed by vacuum evaporating the overcoat layer onto the photosensitive layer, and heating the support, photosensitive layer and the overcoat layer to form wrinkled-shaped convexities and concavities.